Various types of polyethylene are known in the art. Low density polyethylene ("LDPE") is generally prepared at high pressure using free radical initiators and typically has a density in the range of 0.915-0.940 g/cm.sup.3. LDPE is also known as "branched" polyethylene because of the relatively large number of long chain branches extending from the main polymer backbone.
High density polyethylene ("HDPE") usually has a density in the range of greater than 0.940 to 0.960 g/cm.sup.3. HDPE is prepared using a coordination catalyst, e.g. Ziegler-Natta type catalysts, at low or moderate pressures, but sometimes at high pressure. BDPE is generally linear without any substantial side chain branching. HDPE is a substantially crystalline polymer.
Linear, low density polyethylene ("LLDPE") is generally prepared in the same manner as HDPE, but incorporates a relatively minor amount of an .alpha.-olefin comonomer such as butene, hexene or octene to introduce enough short chain branches into the otherwise linear polymer to reduce the density of the resultant polymer into the range of that of LDPE. The coordination catalysts used to interpolymerize ethylene and the .alpha.-olefin generally produce an LLDPE with a relatively broad weight molecular weight distribution, i.e., Mw/Mn greater than about 3. Such LLDPE's also have relatively broad composition distribution in that the proportion of .alpha.-olefin comonomer molecules incorporated into the polymer molecules varies. Generally, the lower molecular weight polymer molecules contain a relatively higher proportion of the .alpha.-olefin comonomer than the higher molecular weight polymer molecules.
A polyethylene such as LLDPE having a broad molecular weight distribution is undesirable in many respects, depending on the desired end use application. For example, LLDPE resins known in the prior art containing relatively high molecular weight molecules are subject to orientation which results in anisotropic properties in the machine versus transverse direction of a fabrication process. On the other hand, resins containing relatively lower molecular weight molecules, in which the comonomer is invariably concentrated, tend to exhibit high block and tackiness. These lower molecular weight, highly branched molecules interfere with the proper function of certain additives compounded in the resin, increase the percentage of extractable polymer, and increase fouling in the polymerization plant. The relatively high .alpha.-olefin comonomer content of these low molecular weight polymer molecules causes such polymer molecules to be generally amorphous and to exude to the surface of fabricated parts, thereby producing an undesirable sticky surface.
Prior art polyethylenes such as LLDPE also generally tend to have a very broad, nonuniform distribution of comonomer content, i.e. some polymer molecules have a relatively high .alpha.-olefin comonomer content while others have a relatively low content. Generally, the polymer molecules of low comonomer content are relatively more crystalline and have a high melting temperature, whereas the high comonomer content polymer molecules are more amorphous and melt at a lower temperature. The presence of a higher melting component is disadvantageous in many applications, for example where softness or clarity is desired. On the other hand, the presence of a lower melting component frequently results in a high quantity of extractables, which limit food contact applications.
Prior art blends of polyethylenes designed to improve one or more of the properties of the blend relative to its blend components or prior art polyethylene have also suffered from the drawbacks mentioned above. For example, incorporating a blend component with a high average comonomer content to reduce crystallizability generally results in an increase of extractables and adversely affects other properties so that the full advantage of the blend is not realized. Thus, there is a need to provide ethylene interpolymer blends with superior properties and in which the full advantages of blending may be realized.